Simone Biles will not be defending her gold medal in the all-around final at the Tokyo Games -- the Olympics superstar has backed out of the competition to continue to focus on her mental health.
24-year-old Biles -- who withdrew from the team finals on Tuesday -- underwent further medical evaluation following the event ... and it was determined it was best she sat out Thursday's finals, USA Gymnastics said in a statement.
Biles was the heavy favorite to win the event ... which would have secured the 5th gold medal of her career.
It may not be the end of Biles' Olympic run, however -- there's still a chance she returns to compete in the individual events next week ... but the team says it all depends on the results of her daily evaluations.
"We wholeheartedly support Simone's decision and applaud her bravery in prioritizing her well-being," USAG said. "Her courage shows, yet again, why she is a role model for so many."
Biles absolutely crushed the competition at the 2016 Rio Games ... posting a 62.366 score, with teammate Aly Raisman taking silver with a 60.607.
Jade Carey -- who placed 9th in qualifiers -- will be Biles' replacement.